In the manufacture of printed circuit cards and boards, a dielectric sheet material is employed as the substrate. A conductive circuit pattern is provided on one or both of the major surfaces of the substrate. The conductive pattern can be formed on the surface of the substrate using a variety of known techniques. These known techniques include the subtractive technique, where a layer of for example copper is etched to form the desired circuit pattern, the EDB (electroless direct bond) technique, where copper is electrolessly plated directly on the surface of the substrate in the desired pattern, and the peel-apart technique, where the desired circuit pattern is plated up from a thin layer of peeled-apart copper. Since the substrate employed is a dielectric, when it is desired to plate directly on the surface of the substrate, various techniques for seeding or catalyzing the substrate are used.
Furthermore, in various situations, it is desirable to selectively plate on the metallic surfaces (usually copper) as opposed to the dielectric surfaces of the substrate. This is especially true for plating copper areas that are to be used for electrical connection. For example, it is common practice to overplate copper lines with a barrier layer, typically a metal such as nickel followed by a second overplating with a precious metal such as gold, palladium or rhodium. Examples of such processes are disclosed in U.S. Pat. Nos. 4,940,181 and 5,235,139, disclosures of which are incorporated herein by reference.
However, there is a tendency for the nickel and/or precious metal to plate not only on the already present circuit lines, but also to deposit on a portion of the dielectric substrate or insulator located between lines. This problem is especially pronounced when dealing with very fine lines that are only separated by very small intervals. For example, circuit boards having surface conductive paths whose spacing is 50 microns or below have a tendency to suffer from bridging or short circuiting due to the subsequent plating of the nickel and/or precious metal. It would therefore be desirable to provide a process for plating only already circuitized lines whereby the problem of bridging is significantly reduced if not entirely eliminated.